The present invention is directed to liquid crystal display devices and more particularly, to a display system comprising an apparatus for providing front illumination to a reflective liquid crystal display.
Liquid crystal displays (LCD""s) are commonly used in portable computer systems, cellular telephones, pagers and other portable electronic devices in order to display information to the user. An LCD requires a source of light for operation because the LCD is effectively a light valve, selectively allowing transmission of light in one state and blocking transmission of light in a second state. LCD""s can be generally broken down into two broad categories, reflective liquid crystal displays and transmissive liquid crystal displays. As the name implies, in a transmissive liquid crystal display, light, typically from a backlighting device, is selectively transmitted through the liquid crystal display for viewing by the user positioned on the side of the LCD opposite the backlight. A reflective liquid crystal display, on the other hand, receives light from the front surface and selectively reflects it back through the front surface of the LCD for viewing by the user. Reflective liquid crystal displays have advantages over transmissive liquid crystal displays primarily in that, except in very low light conditions, they can use ambient light rather than requiring a backlighting apparatus. Accordingly, portable devices using reflective liquid crystal displays have substantially lower power consumption than an equivalent device having a backlighted transmissive liquid crystal display.
A conventional method of providing artificial illumination for a reflective liquid crystal microdisplay, used in a virtual display, involves use of a polarizing beam splitting cube. Front illumination of a reflective liquid crystal display by means of a polarizing beam splitting cube, however, is difficult to implement in a compact portable device because the thickness of the polarizing beam splitting cube is typically at least as thick as the liquid crystal display is wide along its longest dimension. The relatively thick polarizing beam splitting cube also increases the complexity of the projection or viewing optics as these must, of necessity, have a retrofocus distance at least as great as the thickness of the polarizing beam splitting cube through which the projection or viewing optics must focus. Accordingly, front illumination of a liquid crystal cell by means of a polarizing beam splitting cube is impractical for most portable electronic devices. Accordingly, what is needed is a compact thin apparatus for providing front illumination for a reflective liquid crystal display.